Not applicable.
Not applicable.
The present invention relates generally to variable switches commonly referred to as xe2x80x9cdimmer switchesxe2x80x9d, and more particularly to a dimmer switch with reduced electronic noise.
It is often desirable to adjust the output of a lamp or similar electronic device. Many conventional dimmers use a switching circuit to vary the conduction angle of an alternating current (xe2x80x9cACxe2x80x9d) signal. Switching circuits often use solid-state semiconductor switching devices such as a triac or a silicon-controlled rectifier (xe2x80x9cSCRxe2x80x9d) to vary the power delivered to the load (e.g. incandescent lamp). However, in AC dimmers the switching devices are typically turned on and off very fast. This sharp transition generates radio frequency interference (xe2x80x9cRFIxe2x80x9d) or xe2x80x9cnoisexe2x80x9d. Such noise often interferes with radio reception, and some commercially available AC dimmer switches even warn against using the AC dimmer switch within a certain distance of a radio receiver.
Furthermore, triacs have a turn-on voltage drop, typically about 0.50-0.75 V that can generate significant heat and require a large heat sink for high-power loads. For example, a triac drawing 6 amps will generate about 3-4.5 W of power. The turn-on voltage can also create hysterisis in the turn-on/turn-off behavior of the switch. That is, in order to dim a lamp to nearly off, the triac must first be turned on above its turn-on voltage, and then the dimmer switch operating point can be reduced to the desired low level. Such behavior precludes having the dimmer pre-set to operate at a low level when the dimmer switch is activated, and the light usually snaps on at a relatively high level. The turn-on characteristics of conventional triac- or SCR-based AC dimmers can also cause flickering near the turn-on voltage, as well as a discontinuity (xe2x80x9cstepxe2x80x9d) in the brightness of a lamp.
Therefore, it is desirable to provide a dimmer switch with sufficiently low RFI to be used near a radio receiver. It is further desirable that the dimmer switch be efficient and not require excessive heat sinking, and allow a pre-set turn-on low-output operating point.
A direct-current switch is used in a dimmer switch to provide a low-noise variable switch. A rectifier bridge converts AC line voltage to pulsating DC. The pulsating DC is provided to a load through a series DC switch, in one case the DC switch is a FET with a series resistance less that 0.1 ohms when on. In other embodiments the series resistance is as low as 0.001 ohms. The DC switch is turned on during a pulse signal generated by a comparator. The width (period) of the pulse signal is basically the time a ramp voltage input to the comparator exceeds a set-point voltage input to the comparator. The ramp voltage is generated by an integrator using capacitive feedback. The integrator is re-set at each zero crossing of the line voltage when the pulsating DC voltage drops below a minimum supply voltage of the integrator and the capacitor discharges. The pulse output of the comparator is provided to the input of the FET through a series resistor. The combination of the input capacitance of the FET and the series resistor avoids a sharp turn-on characteristic of the FET, thus reducing the generation of electromagnetic noise.
In a further embodiment, a DC dimmer circuit is combined with a toggle to turn the load on and off at a pre-selected level. The DC dimmer circuit allows essentially full-range control because of the small voltage drop across the DC switch. Thus, the DC dimmer can be turned on at very low pre-set levels. Furthermore, because of the low power dissipation in the DC switch, relatively little heat sinking is required.
In a further embodiment, a DC dimmer switch is combined with a radio receiver into a radio receiver product. The DC dimmer switch is essentially noiseless. A detector, such as a passive infrared detector, may be further combined with the toggle to turn the load on and off at a pre-selected level upon a signal from the detector, such as when a person walks into a room or waves a hand in front of the detector.